Kawasaki Disease (KD) is an infectious disease of unknown cause that can be difficult to diagnose and treat and can lead to lifelong heart disease or death from coronary artery (CA) aneurysms in previously healthy children. Our long-term research goal is to identify the pathogenesis of KD, so that a diagnostic test can be developed and improved therapies based on the molecular pathogenesis be designed. A paradigm of KD arteriopathy has held that KD is a self-limited acute vasculitis lasting 2-3 months after onset, and that arterial occlusion in KD was the result of inactive, healed scar. We recently performed an extensive light and electron microscopic pathologic study of coronary arteriopathy in 41 KD cases that significantly challenges this model. We identified 3 linked pathologic processes in KD arteriopathy: necrotizing arteritis (NA), subacute/chronic vasculitis (SA/C), and luminal myofibroblastic proliferation (LMP). NA was the only self-limited process and was complete by 2 weeks after fever onset. SA/C and LMP began in the first 2 weeks but could persist for years. LMP is a medial smooth muscle cell-derived myofibroblastic proliferative process that can lead to progressive arterial stenosis, and may represent aberrant wound healing. Our specific aims propose paradigm shifts for KD research that have significant clinical implications for KD patients. Identification of dysregulated molecular pathways in KD arteriopathy can lead to target-directed rather than empiric therapies. Children with chronic KD arteriopathy can have persistent or even worsening CA disease over time. While the prior paradigm indicated that their vasculitis resolved at ~2 months after onset, we showed persistence of SA/C, and identifying the molecular basis of their dysregulated immune response could result in a new approach to treatment. The discovery that LMP is an active proliferative process instead of scar leads to the need to understand its pathogenesis and develop therapies to prevent or reduce this serious KD complication. Previous biomarker studies of KD have focused primarily on inflammatory molecules, but markers of cardiovascular stress would likely better distinguish KD patients from children with febrile illnesses in the differential diagnosis. In this proposal, we will: 1) identiy the molecular pathogenesis of acute KD arteriopathy; 2) determine the molecular pathogenesis of immune dysregulation in chronic KD arteriopathy, and 3) identify cardiovascular damage/stress induced molecules that are elevated in the plasma of KD children with CA dilation compared with febrile control children. The results of these studies will provide insights into the molecula pathogenesis of KD arteriopathy and identify new diagnostic tools and therapeutic targets.